As far as I remember it was supposed to keep 'grit' out of the compression chamber. Who rubs their break-barrel in grit I wonder? As it is not fine enough to keep dust out it would have had a limited effect in preventing cylinder wear, and also it would get in the way of the air-flow so we can conclude it was a gimmick. I bet the rifle works better with it removed. BSA likes putting things in the transfer port, its a sort of fetish. And sadly they invented the roller breech about a hundred years before they decided to use it.
Possibly the Cadet was designed for trench warfare in Mesopotamia?